Q. David Bowers (edited and updated by Mike Sherman): In 1866, the Liberty Half Eagle was modified by adding the motto IN GOD WE TRUST on a ribbon above the eagle on the reverse. Apart from this, the motif with Liberty Head on the obverse and perched eagle on the reverse is the same used in earlier years. Mintage of the with-motto style was continuous at the Philadelphia Mint, with additional coins being made at San Francisco, Carson City, New Orleans and in 1906 and 1907, at the new Denver Mint. Combined with the earlier “no motto” style, the Liberty Half eagle is the only coin to be struck at all seven of the “traditional” U.S. mints.
Due to the high original mintages of many issues (particularly after 1880), obtaining a “with motto” half eagle should present no difficulties in any grade, up to and including MS-65. MS-66s are available, though scarcer. Above MS-66, the supply thins considerably. As a date, the only “show stopper” is the Philadelphia issue of 1875. With an original mintage of only 200 pieces, fewer than five have survived. Philadelphia minted Proofs in all years, and are rare.
-- Reprinted with permission from "United States Coins by Design Types - An Action Guide for the Collector and Investor" by Q. David Bowers